Covariance, contravariance and invariance describe how the existing type inheritance hierarchy changes when subjected to some transformation (such as usage within generics). If the transformation keeps the ordering of the original hierarchy, it is "covariant". If it reverses it, it is "contravariant". If it breaks it, it is "invariant".

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Subtypes and supertypes are a common part of modern programming. Covariance and contravariance tell us where subtypes are accepted and where supertypes are accepted in place of the original type.

Variance is a topic that comes up fairly often in type systems and can be a bit confusing the first time you hear it. Let’s walk through each form of variance.